Flat panel LCDs use two transparent glass plates as substrates. In a typical embodiment, such as one set forth in U.S. Pat. No. 5,503,952 issued Apr. 2, 1996 to Suzuki et al., a set of electrical traces is sputtered in a pattern of parallel lines that form a first set of conductive traces. A second substrate is similarly coated with a set of traces having a transparent conductive coating. Coatings are applied and the surfaces rubbed to orient liquid crystals. The two substrates are spaced apart and the space between the two substrates is filled with a liquid crystal material. Pairs of conductors from either set are selected and energized to alter the optical transmission properties of the liquid crystal material. Such displays are expensive.
Fabrication of flexible, electronically written display sheets using conventional nematic liquid crystals materials is disclosed in U.S. Pat. No. 4,435,047 issued Mar. 6, 1984 to Fergason. A first sheet has transparent indium-tin-oxide (ITO) conductive areas and a second sheet has electrically conductive inks printed on display areas. The sheets can be thin glass, but in practice have been formed of polyester. A dispersion of liquid crystal material in a binder is coated on the first sheet, and the second sheet is bonded to the liquid crystal material. Electrical potential is applied to opposing conductive areas to operate on the liquid crystal material and expose display areas. The display uses nematic liquid crystal materials, which ceases to present an image when de-energized. Privacy windows are created from such materials using the scattering properties of conventional nematic liquid crystals. Nematic liquid crystals require continuous electrical drive to remain transparent.
U.S. Pat. No. 5,437,811 issued Aug. 1, 1995 to Doane et al. discloses a light-modulating cell having a chiral nematic liquid crystal (cholesteric liquid crystal) in polymeric domains contained by conventional patterned glass substrates. The chiral nematic liquid crystal has the property of being driven between a planar state reflecting a specific visible wavelength of light and a light scattering focal conic state. Chiral nematic material has two stable slates and can maintain one of the stable states in the absence of an electric field. Consequently, chiral nematic displays have no limit on the number of lines that can be addressed. U.S. Pat. No. 5,251,048 issued Oct. 5, 1993 to Doane et al., and U.S. Pat. No. 5,644,330 issued Jul. 1, 1997 to Catchpole et al. disclose various driving methods to switch chiral nematic materials between its stable states. However, the update rate of these displays is far too slow for most practical applications. Typically, the update rate was about 10-40 milliseconds per line. It would take a 10-40 seconds to update a 1000 line display.
U.S. Pat. No. 5,748,277 issued May 5, 1998 to Huang et al., and U.S. Pat. No. 6,154,190 issued Nov. 28, 2000 to Yang et al. disclose fast driving schemes for chiral nematic displays, which are called dynamic driving schemes. The dynamic driving schemes generally comprise a preparation step, a selection step, and an evolution step. The voltage amplitude and the duration of the preparation step need to be sufficient to cause the complete transformation of the material to the homeotropic texture. U.S. Pat. No. 6,154,190 also discloses a two-phase drive scheme having a preparation step and a selection step. According to this patent, a preparation voltage is applied for a sufficient duration to obtain the focal conic texture.
U.S. Pat. No. 5,661,533 issued Aug. 26, 1997 to Wu et al. discloses a drive method comprising an initialization step and an addressing step. During the initialization step, the liquid crystal material is initialized into a homeotropic (called a nematic phase in the '533 patent) texture, and subsequently driven to a focal conic texture. This focal conic state then serves as a known reference for subsequent driving. It is suggested that this unique process of initialization of the focal conic state makes the switching of material from the focal conic state to the planar state faster than directly writing the material to the focal conic state with a single step. A similar idea has been proposed in a paper (Hashimoto et al. “Reflective Color display using cholesteric liquid crystals”, SID Digest 1998, pp.897-900). According to this paper, all pixels are first set into the focal conic state simultaneously, then each pixel is selectively driven to the planar state or remain in the focal conic state. Other similar ideas can be found in “Simple driving methods for cholesteric reflective LCDs” by Sorokin in Asia Display 1998, pp.749-752, which suggests to switch all pixels into the transient focal conic homeotropic state by the effective voltage U/√{square root over (3)} in the preparation phase. U.S. Pat. No. 6,268,839 issued Jul. 31, 2001 to Yang et al., also proposes a drive method which initializes the liquid crystal to the focal conic or the planar state as a common state for subsequent driving.
U.S. patent application No. 2001/0015723 by Kwok et al., published Aug. 23, 2001, discloses a method in which all the pixels are initially driven to the planar state, and selected pixels are then driven to the focal conic state.
All of the above drive schemes for cholesteric liquid crystal displays fall into two categories. The first catagory is directly writing pixels to a final state independent of initial states such as the ones disclosed in U.S. Pat. Nos. 5,251,048 and 5,644,330, both referenced above. Drive schemes in this first category are slow. They may be made to appear faster by dividing the long drive pulse into a series of short pulses as disclosed in U.S. Pat. No. 6,133,895 issued Oct. 17, 2000 to Huang, and U.S. Pat. No. 6,204,835 issued Mar. 20, 2001 to Yang et al. However, the total drive time for each row is still long. The second category of drive schemes first write all pixels into a common state, then selectively write pixels to different final states. This common state can be a homeotropic state (texture), a focal conic state (texture), a planar state, a transient focal conic homeotropic state, or any gray level state between the focal conic and the planar states. This second category of drive schemes using a common state prior to selection of a final state is conceptually simple. Once all pixels are in a common state, it is easy to subsequently drive pixels into a desired final state.
However, the second category of drive schemes based on driving the pixels to a common state has problems. First, these drive schemes require a substantially long initialization time to prepare the common state. Second, for some of these drive schemes, such as the three phase and five phase dynamic drive schemes described in U.S. Pat. Nos. 5,748,277 and 6,154,190, referenced above, driving all of the pixels to a common homeotropic state can result in the unpleasant appearance of a black bar that sweeps across the image as the display is written. There is a need therefore for an improved fast drive scheme that avoids the problems noted above.